562 there are two general "disorders" that bring about that. the first is not enough 'sunlight' or UV deprivation. the sun actually recharges your body for you and creates a happier and more awake you... go figure..
the second is more of a chemical disorder. it's when your body doesnt produce an even level of chemicals in the bloodstream which can cause fits of dizzyness and major "lag' tiredness..
i had to learn about this in HS and do a report on it. because i slept through my health class every day.